Role of biophysical stimulation in multimodal management of vertebral compression fractures

Raised life expectancy and aging of the general population are associated with an increased concern for fragility fractures due to factors such as osteoporosis, reduced bone density, and an higher risk of falls. Among these, the most frequent are vertebral compression fractures (VCF), which can be clinically occult. Once the diagnosis is made, generally thorough antero-posterior and lateral views of the affected spine at the radiographs, a comprehensive workup to assess the presence of a metabolic bone disease or secondary causes of osteoporosis and bone frailty is required. Treatment uses a multimodal management consisting of a combination of brace, pain management, bone metabolism evaluation, osteoporosis medication and has recently incorporated biophysical stimulation, a noninvasive technique that uses induced electric stimulation to improve bone recovery through the direct and indirect upregulation of bone morphogenic proteins, stimulating bone formation and remodeling. It contributes to the effectiveness of the therapy, promoting accelerated healing, supporting the reduction of bed rest and pain medications, improving patients’ quality of life, and reducing the risk to undergo surgery in patients affected by VCFs. Therefore, the aim of this review is to outline the fundamental concepts of multimodal treatment for VCF, as well as the present function and significance of biophysical stimulation in the treatment of VCF patients.


Introduction
Raised life expectancy and aging of the general population are associated with an increased risk of fractures.Among those typically occurring in the older adults, fragility fractures represent a rising concern for public health, usually occurring on a pathologically weakened bone [1].Osteoporosis and reduction in bone density, together with the increase in fall events are typically observed in the older adults, because of the higher occurrence of sarcopenia, neuromuscular pathologies, or cognitive impairment [2].In the United States, it was estimated that 54 million adults over the age of 50 have a decreased bone mass [3].Among these, 40-50% of women over 50 years of age will encounter fragility fractures during their lifetime because of the intrinsic increased risk of osteoporosis and poor bone metabolism occurring after menopause [4,5].
Among fragility fractures, the most frequent are vertebral compression fractures (VCF), which primarily occurs in the older adults as a result of a low-energy trauma in patients already suffering from osteoporosis, even though, in the case of severe osteoporosis, VCF can occur while doing something simple like coughing or sneezing [6].
Most VCF are clinically occult; in fact, about 1 in 3 vertebral fragility fractures are identified clinically, with only a small percentage requiring hospitalization [5].Back pain is the main symptom, occurring in 85% of patients.A VCF is identified with a reduction in the height of the vertebral body by at least 15-20% and most commonly regards the thoracolumbar junction.
Diagnostic imaging includes antero-posterior and lateral views of the affected spine at the radiographs.To meet radiographic criteria, vertebral body height should decrease by at least 20% on the anterior wall compared to the posterior wall, or by at least 4 millimeters from baseline height (Fig. 1-A) [7].However, the most significant challenge is to distinguish between recent vs old fractures (timing of the fracture).Considering that recent fractures exhibit bone edema, magnetic resonance imaging (MRI) is the most useful tool to define the time of fracture, and can recognize pathologic fractures in case of metastatic or primary tumor involvement of the bone (Fig. 1-B,C).MRI is also a useful tool to monitor the healing of the fracture because bone edema regresses during the fracture healing.
CT scan is an effective screening tool for vertebral fractures.Most recently, Dual Energy-CT (Fig. 2) has been proposed as an alternate imaging exam for the assessment of VCF because of its ability to outline the presence of bone edema, giving information about bone healing and time from fracture.Also, it can be performed also in patients with pacemakers or other implanted devices, in which the performance of MRI is contraindicated [8].
In a patient with the diagnosis of VCF a comprehensive workup to assess the presence of a metabolic bone disease or secondary causes of osteoporosis and bone frailty is required.Dual-energy x-ray absorptiometry at the lumbar spine and proximal femur is generally performed to check for osteoporosis and risk of fracture.Laboratory evaluation including a complete blood count; complete metabolic workout with kidney function; measurement of erythrocyte sedimentation rate and thyroid-stimulating hormone, 25-hydroxyvitamin D, parathyroid hormone, and C-reactive protein levels is assessed [5].Once the diagnosis is made and the patient is evaluated for risk factors, treatment is planned, which at present requires an integrated envision and management of the different aspects of the patient's bone health, both metabolic and biomechanics [9].Multimodal therapy of VCF in several Western Countries, has recently incorporated Biophysical Stimulation.
The aim of this review is therefore to describe the main principles of multimodal treatment of VCF, outlining the current role and contribution of biophysical stimulation to the management of VCF patients.Current evidence from experimental in vitro and in vivo studies, and most recent clinical trials on the topic, will be categorized and reported.

Multimodal management of patients with VFC
Multimodal management (MM) of patients with VCFs involves several specialists to address the different aspects of patients' disease, including orthopedic surgeons, physical therapists, bone metabolism specialists, and in some cases pain specialists.The mainstays of current standards in MM of VCF include a combination of immobilization with a brace, pain management, exclusion of potential secondary forms of bone metabolic diseases, and prescription of osteoporosis medication (Fig. 3) [10].Therapy is aimed at the promotion of the healing of fractures, and this may be supported by biophysical stimulation [11].Patients failing conservative treatment often require surgery, which may be of cementoplasty, or a spinal instrumentation, performed by a standard or minimally invasive approach [12].

Etiological evaluation and pharmacological treatment
Appropriate treatment of patients with VCF requires the determination of the presence of an underlying metabolic bone disease.Therefore, a comprehensive workup, most of the time by blood and urine samples harvest analysis is the beginning of the management of the patient.In most patients, primary osteoporosis will be the principal cause of the VCF.However, at present time, secondary forms of osteoporosis include hypogonadism, endocrine disorders, gastrointestinal diseases, transplantation, genetic disorders, and use of medications, some of which are frequently encountered also in male patients.In the case of diagnosis of a secondary metabolic bone disease, targeted therapy is started [13].One of the most common forms of osteoporosis is secondary to vitamin D shortage; this condition is becoming endemic in western countries because of the reduced exposure to sun and dietary intake [14]; vitamin D optimization (400-1000 IU) and calcium (1500 mg/d) intake is necessary as the use of these supplements determines a significant reduction in the overall number of fractures reaching 15% [13,15].
VCF by itself is a sign of bone weakness, and it requires a treatment to reverse the overall patient status and to prevent additional fractures from occurring; moreover, drugs acting on bone metabolism can play a role in patients' pain management [16].Current drugs include bisphosphonates, calcitonin, estrogen, selective estrogen receptor modulators, parathyroid hormone, and receptor activators of nuclear factor kappa-B ligand inhibitor [17].Bisphosphonates are by far the most prescribed agents, and these are the first line of treatment in symptomatic patients; these act by osteoclast inhibition, leading to reduced bone turnover, increased bone mass, and improved mineralization [18,19].Teriparatide shares the first active 34 amino acids of the N-terminal end of the PTH molecule and can increase bone formation when administered intermittently.It reduces back pain, enhances bone mineral density, and decreases the probability of a later fracture; however, it can be prescribed in selected patients, and it cannot be administered in patients with a history of malignancy [20].
Calcitonin was used in the past because of its anabolic impact on bone, and it also provided pain relief through modulating nociception in the central nervous system [9].The most recent drug for the management of vertebral fractures is Romosozumab, the first anti-sclerostin humanized monoclonal antibody whose effects on bone metabolism and risk of fractures have been recently reported in clinical trials.By binding to sclerostin, permitting the engagement of Wnt ligands with their co-receptors, results in increased bone mineral density (BMD) [21].Studies confirmed the ability of Romosozumab to decrease the risk of fragility fracture to develop [22,23].
Pain control is crucial in conservative management.Analgesics are the first line in vertebral compression fracture treatment, generally administrating nonsteroidal anti-inflammatory drugs (NSAIDs).Opioids such as oxycodone can be combined with paracetamol for patients who do not respond well to first-line pain relievers [16].Among antidepressants, Inhibitors of the Reuptake of Serotonin (SSRIs) can be used short-term in association with standard therapy to control pain in the acute phase in fragile patients; its main drawbacks are the risk of falling at high dosage and light sedation in chronic assumption [24].Serotonin also plays an important role centrally in functions such as appetite, sleep, sex, and temperature, and recent evidence shows that it may be an important regulatory agent in bone metabolism, increasing bone mass.Furthermore, pregabalin has demonstrated efficacy in pain management, with studies showing a reduction on reducing opioid consumption in the first 24 h post-surgery by 20-62% when used in postoperative analgesia and non-inferior rates of surgery for nonunion [25].

Rest and Brace and Physical Therapy
Soon after a symptomatic VCF, bed rest is advisable because it decreases the axial loads over the fracture site and decreases pain.However, bed rest might result in muscle and bone weakness, pressure sores, and deep vein thrombosis; therefore, as soon as the patients feel better, usually after the first two to four weeks, it is suggested to encourage mobility by wearing a spinal brace.The use of a brace promotes healing by reducing the motion at the fracture site, and it decreases pain by sharing the load and correcting spinal posture.It is deemed that these  tools may contrast the kyphotic deformity to develop at the fracture site [26].When used, a spinal orthosis is recommended in patients with VCF for six to eight weeks approximately.Depending on the location and severity of the fracture, several brace types are available.Thoracolumbar (TLO) brace can be used to treat fractures at the thoracolumbar region, such as the Jewitt, cruciform anterior spinal hyperextension, and the Taylor brace.Thoracolumbar sacral orthoses (TLSO) refer to braces that go all the way to the sacrum, and these are prescribed for fractures at the lumbar vertebral bodies [27].Braces may also offer different support if rigid, semi-rigid, or soft.Due to its higher discomfort and decreased compliance, the use of rigid bracing has reduced over time [28].Some patients may benefit from custom made braces to improve wearability, increase the structural support or correct actual deformities as in the case of antigravity braces to correct or prevent kyphotic deformity.
Since braces can determine core muscle weakness and skin complications, patients should be followed up and an appropriate rehabilitation program should follow [29].Current protocols for the treatment of osteoporosis include at least 30 min treadmill or cycle daily.These training exercises are not possible in patients with acute or subacute VCF because of pain and the increased risk of vertebral body collapse and segmental kyphosis.However, after two months from VCF, return to cycle and physical therapy is appropriate: core strengthening exercises reduce chronic pain, enhance posture and gait, improve the quality of life, and strengthen the back extensors after the initial pain has decreased.Additionally, contrasting the development of sarcopenia, it could decrease the risk of subsequent falls and fractures to occur [16].

Biophysical stimulation
Biophysical stimulation is a noninvasive technique that uses induced electric stimulation to improve bone recovery through the direct and indirect upregulation of bone morphogenic proteins, stimulating bone formation and remodeling.An observational study showed a positive role in osteoblastic function, an inhibition of osteoclast activity, and a clear role in contrasting bone edema, the main characteristic of acute VCFs [30].
Three methods of biophysical stimulation of osteogenesis have been developed so far, and these include continuous electrical currents directly applied to the bone tissue through implanted electrodes (DC, faradic systems), alternating electrical currents induced externally using pulsed electromagnetic fields in the bone tissue (PEMF, inductive systems), and the alternating electrical currents induced externally using capacitively coupled electric fields (CCEF, capacitive systems) (Fig. 4) [31].
While faradic systems require surgical intervention, albeit minimal, to position the electrodes that release the current at the fracture site, inductive and capacitive systems are non-invasive.The mechanisms of action through which the electrical current applied to the bone tissue with the three methods described above promote osteogenesis are different.
In faradic systems, the direct action of the continuous electrical current manifests with both purely electrical phenomena, which interfere with the dynamics of ions at the fracture site, and chemical processes that lead to a reduction in local oxygen tension and a modest Fig. 4. Schematic representation of the three methods of biophysical stimulation: Direct current or DC (A) where a cathode is implanted directly at the fracture site to generate an electric field; Capacitive coupling or CC (B) where two electrodes are located on the two sides of the fracture at skin level and the electric field is generated from an external power source attached to the electrodes; Inductive coupling (IC) where a single electromagnetic current carrying coil is placed on the skin over the fracture site and the electric field is generated from an external power source.increase in pH.Faradic systems apply higher electrical voltages to the bone tissue compared to inductive or capacitive systems, but their use is solely experimental, and these have not been introduced into clinical practice [32].
The biological activity of PEMF can be explained both through the time-varying magnetic component and the induced electric component, namely the electric field.These signals have complex waveforms, with predominant spectral content ranging from tens of Hertz to tens of thousands of Hertz.The main interaction sites of PEMF are believed to reside at the cell membrane, particularly involving calcium receptors and channels.In vitro experiments have shown that exposure to PEMF promotes the proliferation of human osteoblasts, as well as neoangiogenesis of endothelial cells in cultures.In vivo studies observed an increase in the formation of bone tissue and a reduced time for consolidation of experimental bone fractures.Capacitive systems determine biological effects solely because of the presence of the timevarying electric field.Similar to PEMF these act at the cell membrane and increase osteoblastic activity [33].
In these methods, the waveform of the signal and the intensity, frequency, and duration of the electrical or magnetic stimulation play a crucial role in achieving the desired therapeutic response.The electrical voltage applied can vary between 1 and 10 V, with frequencies ranging from 20 to 200 kHz, and the electric field within the tissue is typically between 1 and 100 mV/cm.The optimal values vary depending on the specific method used [34].
At present, two different types of electrical stimulation are used in clinical settings to promote bone healing: inductive system (PEMF pulsed electromagnetic fields), and capacitive systems (CCEF capacitively coupled electric field) which exploit the properties of the electric field.Recently, a low energy ultrasound-based system to deliver electric fields has been developed and introduced into clinical practice (LIPUS=Low intensity pulsed ultrasound system) [35].Three biophysical stimulation medical devices produced by IGEA®, EBI® and ORTHOFIX® are available on the market; these non-invasive devices use alternating currents in current-carrying coils on the skin over the fusion site to deliver pulsed electromagnetic field stimulation (PEMF).The pulsed electromagnetic field created by these currents stimulates vascularization, osteoblast migration, an increase in matrix production, and mineralization of developing bone [36].Because of the morphology of coils and discomfort, for spinal applications, capacitive system is preferred to deliver CCEF at the fracture site [37].In vitro, in vivo, and clinical studies support the use of PEMF and CCEF to promote bone healing, whose results are reported below.

In vitro studies
Physical stimulation is recognized and transferred to the various metabolic pathways at the cell membrane level.The inductive system determines the release of calcium ions (Ca++) from the smooth endoplasmic reticulum, while with the capacitive system there is the opening of the membrane channels for the voltage-dependent Ca+ + (Fig. 5).
PEMFs directly control signal transduction by releasing Ca2 + intracellularly, which determines a series of enzymatic reactions resulting in gene transcription and cell proliferation through the synthesis of growth factors such as BMPs, TGF-β and various matrix proteins which lead to an acceleration of the reparation and biomineralization processes [38,39,40].
Exposure of bone cell cultures to PEMF stimulates the production and release of growth factors belonging to the TGF-β/BMPs family, which is complemented by a positive anabolic effect, osteogenic differentiation, and increased cell proliferation [41,42,43].It also promotes osteogeneic differentiation of bone marrow and adipose tissue harvested mesenchymal stem cells [44].It has been observed that electrical stimulation promotes the endogenous synthesis of BMP-2; its effect can be maintained over time because target tissues are exposed for the entire duration of the healing process, and the physical stimulus acts as an endogenous modulator, keeping its activity over time.
The shape of the electromagnetic field is one of the major determinants of cellular effects: continuous low-intensity static magnetic fields (SMF) negatively influence the differentiation and proliferation of osteoblast cell cultures [45] and increase human osteoclast differentiation; on the contrary, pulsed electromagnetic fields (PEMF) minimally impacts osteoclasts differentiation but seem to have an impact on differentiated bone cells reducing osteoclastic resorption and promoting osteoblastic bone deposition [20].In particular, human osteoblasts isolated from bone tissue samples increase their proliferative activity when exposed to a pulsed electromagnetic field and this effect appears to A. Di Martino et al.
Other determinants of the final effect include the entity of the electromagnetic field and the duration of the exposure.The first molecular proof of biological effects from 60 kHz EF exposures was provided by Bisceglia et al. who conducted a study on Human liver hepatoma HepG2 cells and Human osteosarcoma SaOS-2 cells [47].An enhancement in alkaline phosphatase enzymatic activity, a marker of bone regeneration, in both cell lines has also been reported [46].As regards duration and timing of exposure, Clark et al. stimulated human calvarial osteoblasts with CCEF administrated for 2 h/day and found an up-regulated mRNA expression of TGF-b family genes (BMP-2 and BMP-4, TGF-b1,-b2 and -b3), fibroblast growth factor (FGF)− 2, osteocalcin (BGP) and alkaline phosphatase [48].Brighton et al. stimulating Cultured MC3T3-E1 bone cells discovered that exposure to capacitive coupling resulted in a significant enhancement in DNA production for any period above thirty minutes by increasing cytosolic Ca2 + , cytoskeletal calmodulin, and prostaglandin E2 all upregulated by the influx of Ca2 + through voltage-gated calcium channels [49].
Pulsed electromagnetic fields (PEMFs) also interact with the adenosine receptors (ARs); therefore, exposure to PEMFs results in a notable increase in the expression of adenosine receptors A2A and A3Ars within different cells or tissues.This increase is associated with a decrease in the levels of several proinflammatory cytokines, particularly A2A and A3Ars receptors exert their anti-inflammatory effect through the inhibition of prostaglandin PGE2 [50].In vivo tests confirmed the ability of PEMFs to reduce pain and intraarticular inflammation [51].
However, not all forms of electrical stimulation provide the same outcome.Considering direct current (DC), capacitive coupling (CC), pulsed electromagnetic field (PEMF) and degenerate wave (DW), DW showed the greatest proliferative and least apoptotic and cytotoxic effects on cellular activities.More cells invaded collagen as a result of CC and DW, which also produced more MMP-2 and MT1-MMP [52].(Tables 1-3).

In vivo studies
Since 1983, numerous studies have been conducted to test the efficacy of biophysical stimulation in animal models.First, Brighton et al. demonstrated on Sprague-Dawley rat model that capacitive system can prevent or reverse disuse osteoporosis in the rat limb with a doseresponse effect [49,[56][57][58].They also showed how 8 weeks of stimulation with CCEF reversed the castration-induced osteoporosis in the lumbar vertebrae and restored bone mass per unit of volume in rats models [59].Similar findings have been observed by Carter et al. [60] and McLeod et al. [61] which found a10 per cent mean increase in the bone area stimulated with CCEF and a reduction of osteopenia, particularly expressed on the frequency of 15 hertz.The anti-inflammatory effect of PEMFs was also studied in Sprague Dawley Rats models, proving their inhibitory effect on acute inflammatory cytokine expression, resulting from the downregulation of FGF-1 and upregulating MMP-2 compared to the control group [62].In addition, the effects of electrical stimulation were also evaluated when administrated along with a tissue respiration stimulating agent (Solcoseryl) through the study of dental implants and their osseointegration into the surrounding bone.In this context, an increased levels of bone formation on microscopic observation, bone contact ratio, bone surface area ratio, and the level of removal torque of the implant have been documented [63].Most recently.Muttini et.al. [37] used an appenninica breed sheep model to study CCEF stimulation for fracture healing, showing that an external fixator used in addition to biophysical stimulation with alternating electricity may accelerate the rate of callus maturation.However, in contrast with the proven utility of CCEF in healing delayed or nonunions, its use in treating regenerating bone from distraction osteogenesis appears to be contraindicated, showing a reduction in the rate of recovery of torsional strength and a subsequent delay in bone strength restoration [64].

Clinical studies
Clinicians successfully employed biophysical stimulation to support reparative osteogenesis, and post-traumatic fracture healing in VCFs [69].Clinical studies showed the beneficial impact on pain through a decreased requirement of pain medications.CCEFs have the theoretical advantage, compared to pharmacological administration, of producing locally a constant increase in the concentration of growth factors without the use of large initial dosages, which may accompany local or systemic toxic effects.Rossini et al. [70], in a study conducted on 65 postmenopausal women with radiographically documented multiple vertebral osteoporotic fractures at the thoracolumbar level and chronic pain unsuccessfully treated with NSAIDs for at least 6 months demonstrated that CCEF stimulation was successful in promoting the healing of spinal fusion; moreover, a positive effect on pain without causing adverse events after prolonged use was observed.Patients were treated by Osteospine® (IGEA SpA Carpi (Mo); Italy) for a minimum of 9 h per day for 2 months.The same beneficial effect was demonstrated in patients treated after lumbar spine fusion surgery [71][72][73].In another study, Piazzolla et al. considered 24 patients with acute VCFs, dividing them into two groups conservatively treated with or without CCEF.Patients were managed by Osteospine® (IGEA SpA Carpi (Mo); Italy) for 8 h a day for 3 months.At 90 day follow-up, patients treated by CCEF showed a higher improvement of clinical symptoms and faster fracture healing and BME resolution [30].

Orthopedic surgery
When conservative treatment is unsuccessful, surgery can be required.Surgical management of VCF is rare, and patients and surgeons should be aware of the risk to benefit ratio in this peculiar patients population.When surgery is required, it includes cementoplasty procedures, either vertebroplasty or kyphoplasty, or instrumented spinal fixation [83].In individuals with burst fractures, cementoplasty significantly increases the height of the anterior column, thereby reducing local kyphosis, and reducing pain [84]; despite it is rarely associated with complications, it may promote fractures at the adjacent vertebral bodies [85].Segmental fusion aims for segmental alignment correction; however, it increases the stress on adjacent segments and, in patients   with poor bone quality, implant loosening with segmental kyphosis may occur.In case of major spinal imbalance or neurological deficits, open reduction and spinal fusion are required [86] [87].
In 2016, the North American Spine Society (NASS) issued recommendations regarding the use of electrical stimulation for bone healing, as an aid for spinal fusion.These recommendations highlighted specific clinical scenarios and qualifying criteria for the use of electrical stimulation in the different regions of the spine, including the occipitalcervical, cervical, cervicothoracic, thoracic, thoracolumbar, lumbar, and lumbosacral regions.The guidelines emphasized that electrical stimulation can be considered as an adjunctive therapy for spinal fusion in patients at high risk of developing nonunion.Additionally, the guidelines mentioned the potential utilization of non-invasive electrical stimulation in patients with delayed union in lumbar fusion, either with or without associated risk factors [88].

Conclusions
VCFs are the clinical manifestation most frequently associated with a weakened bone.The formation of a less resistant bone callus, delayed functional recovery, increased risk of subsequent fractures, mobilization of artificial devices and poor osseointegration of prosthetic implants are just a few of the direct or treatment-related complications of fragility fractures.Optimal care in patients with VCF should include the etiological assessment of osteoporosis to rule out secondary causes [17], and most patients benefit from MM.In this setting, biophysical stimulation contributes to the effectiveness of MM of VCFs, promoting accelerated healing, and supporting the reduction of bed rest and pain medications.Economic cost-effective analysis in the long term is required to support their widespread use in clinical practice [36] [81].
In patients with VCF, integrated MM is the gold standard of treatment, consisting of a combination of brace, pain management, bone metabolism evaluation, osteoporosis medication and biophysical stimulation; this treatment improves patients' quality of life, promotes healing, and reduces the risk to undergo surgery in patients affected by VCFs [70,89,73].
Limitations include the possibility of inducing oxidative stress, because of the increased blood flow and circulation carried on by nitric oxide emission which leads to oxygen radicals accumulation and the risk of lowering blood pressure and decreasing heart rate while on PEMF therapy [35],; another limitation is the overall increased costs, not necessarily supported by an improved outcome [79].
Future perspectives may involve the implementation of the computational engineering approach to determine the interaction of different magnetic fields shape and amplitude in vertebrae and other bones with and without the presence of bone cement or instrumentation.Moreover, it could be useful at a cellular level to understand the patterns and predict the rate of resolution of bone edema in fractured vertebras.Finally, it could be useful to improve current devices in the directions of patient specific therapies by matching specific bone edema patterns and fracture patterns to specific deliverable magnetic field amplitudes and shapes [90].

Ethics approval
no ethical approval is required.No statistically significant differences among the three electrical stimulation methods with an overall fusion efficacy of 85% A. Di Martino et al.

Fig. 1 .
Fig. 1. (A) VCF of L4; blue arrow shows the point of decreased height of the anterior portion of the vertebral body compared to the posterior wall; (B) T2-STIR sagittal and (C) T1 sagittal images confirm the presence of the bone edema (arrows) typical of the acute fractures.

Fig. 2 .
Fig. 2. Comparison of the same patient complaining of low back pain and a history of multiple VCFs.On the left, the sagittal CT scan shows multiple VCFs, leaving doubts about which are the acute VCFs.On the right image acquired by the dual energy technique, vertebral bodies are colored in green when bone edema is present, as in acute fractures, and in blue in healed fractures.The picture outlines acute fractures of L2 and L5, and healed fractures of T12, L1, L3, and L4.

Table 1
In vitro studies.

Table 2
In vivo studies.

Table 3
Clinical studies considering vertebral compression fractures and postoperative management after spinal fusion.